1. Field of the Invention
The present invention relates to a camera-ready copy sheet for lithographic printing plates to prevent them from positionally offsetting on plate cylinders in a press.
2. Description of the Related Art
To perform printing with lithographic presses, a lithographic plate is wrapped around a plate cylinder and fixed mechanically.
Conventionally, when lithographic presswork is performed with plates having a non-metallic base on at least the back side as exemplified by a plastic film or paper (coated with a resin on both sides), the softness of the base tends to reduce the accuracy of the positions in which the plate is gripped on to the leading edge of the plate cylinder. If this phenomenon occurs, the accuracy of the plate in the vertical position (i.e., the accuracy in the around-the-cylinder direction) decreases and, in an extreme case, the plate may be fixed slantwise. As a further problem, the friction with the cylinder during printing causes partial distortion in the plate, eventually reducing the accuracy in position relative to the paper to be printed.
Under these circumstances, the use of lithographic plates having a non-metallic base on at least the back side has been limited to the case of short-run work where no problem is caused even if printed pieces have low accuracy in register. On the other hand, exquisite multi-color printing and long-run work on massive presses often fail to achieve the desired color register.
Platemaking and printing operations based on the computer-to-plate (CTP) technology have gained increasing acceptance these days. Compared to the conventional approaches (in which the plate material is subjected to contact exposure with a lith film), the new processes have the advantage of providing good dimensional and positional accuracy for the image (exposure) with respect to the plate material, as well as permitting easy registration in multi-color printing.
Of the two distinctive advantages of the CTP technology, the ease in registration for multi-color printing cannot be realized with lithographic plates based on non-metallic materials such as paper and plastics because they have the problems already described above.
In order to solve the above described problem, it has recently been proposed that a sheeting having an initial elastic modulus of no more than 300 kg/mm2 should be inserted between the press plate and the plate cylinder [see Unexamined Published Japanese Patent Application (kokai) No. 11-20130)]. The sheet has fine particulate matter such as glass beads adhered and fixed thereto so that it has a center-line average roughness Ra of at least 2.
According to the publication, the sheet is prepared by adhering and fixing fine particulate matter such as glass beads onto the surface of a sheet material at high and uniform density. In other words, a highly concentrated liquid dispersion of fine particles is needed to form asperities on the sheeting.
Since the fine particulate matter of the type used in the patent is generally expensive, adhering and fixing it at high and uniform density inevitably increases the cost of the sheet. In addition, the fine particles cannot be easily dispersed in liquid at high concentration and, what is more, agglomeration often occurs in the highly concentrated dispersion and the resulting coarse particles will deteriorate the quality of printed matter.
It is an object of the present invention to provide a camera-ready copy sheet with which lithographic printing plates can be prevented from being positionally offset on a plate cylinder in a press. In the presence of such camera-ready copy sheet, lithographic plates using bases made of non-metallic materials can be applied to multi-color printing or long-run work. In addition, positional offset of plates can be suppressed by merely providing a small amount of particles of at least two different sizes and hardness values on the surface of the copy sheet and this contributes to both cost reduction and easy production. The larger particles have the added advantage of preventing positional offset from occurring between the lithographic plate and the camera-ready copy sheet before printing starts.
According to a first aspect of the present invention, in a camera-ready copy sheet to be inserted between a plate cylinder and a lithographic printing plate at least the back side of which is made of a non-metallic material, said copy sheet having asperities of a predetermined shape on the front side that are urged against the back side of the lithographic printing plate to depress it, the asperities being formed of projections that consist of at least two groups of particles; a larger particle group having a particle size larger than an intermediate between maximum and minimum particle sizes have an average size at least twice an average size of a smaller particle group having a particle size smaller than said intermediate, and the sum per unit area of maximum cross-sectional areas of planes in the particles of the larger particle group that are parallel to the surface of the sheet ranges from 0.1% to 4% of the unit area. This range is preferably in the range of 0.1 to 3.14%.
In the first aspect of the present invention, the sum per unit area of maximum cross-sectional areas of planes in the particles of the smaller particle group that are parallel to the surface of the sheet preferably ranges from 0.1% to 99.9% of the unit area. This range is preferably in the range of 3 to 80%.
In the following description, the larger particle group having the particle size larger than the intermediate are designated xe2x80x9cthe larger particlesxe2x80x9d and the smaller particle group having the particle size smaller than the intermediate are designated xe2x80x9cthe smaller particlesxe2x80x9d, and the proportion of unit area that is occupied by the sum per unit area of maximum cross-sectional areas of planes in the particles that are parallel to the surface of the sheet is referred to as xe2x80x9coccupied area percentagexe2x80x9d.
In the first aspect of the present invention, the larger particles preferably have an average diameter of 5-50 xcexcm. If the larger particles have an average diameter of 50 xcexcm, the smaller particles have an average diameter of no more than 25 xcexcm. If the larger particles have an average diameter of 5 xcexcm, the smaller particles have an average diameter of no more than 2.5 xcexcm. In other words, if the larger particles have an average diameter of 5-50 xcexcm, the smaller particles have an average diameter of no more than 25 xcexcm, preferably in the range of 0.1-25 xcexcm, more preferably in the range of 1.0-25 xcexcm.
In the first aspect of the present invention, the projections that compose the asperities on the camera-ready copy sheet of the invention consist of two or groups of particles, the larger ones being fine inorganic particles such as glass beads or fine particles of comparatively hard polymers such as polystyrene. The smaller particles may be of the same materials as mentioned above; although they may be used in suitably selected amounts, their use is preferably minimized from the viewpoints of cost and production efficiency.
The amount of the smaller particles as relative to the larger particles is preferably in the range of xc2xd to 1000 times, more preferably 1-200 times, and most preferably 1-25 times, the amount of the larger particles.
If the larger particles are distributed coarsely to occupy 0.1-4% of unit area as set forth above, a comparatively small number of the larger particles suffice to achieve positive prevention of positional offset of lithographic plates on the plate cylinder. By using the smaller particles, the lower limit of the percent area that need be occupied by the larger particles to produce the stated effect is reduced to 0.1 (i.e., the number of the large particles that need be used is reduced) and, at the same, this provides ease in separating adjacent camera-ready copy sheets in a stack. If the larger particles occupy 0.1% of the unit area, the remaining part (99.9%) may be occupied by the smaller particles. The stated effect can be achieved if the smaller particles occupy at least 0.1% of the unit area.
To compute the occupied area percentage as defined herein, the surface of a sample is photographed with an optical microscope from right above, the number (n) of projecting particles in a predetermined area S (xcexcm2) is counted, and the occupied area percentage is calculated from the average diameter R (xcexcm) of the particles by the following formula:
Occupied area percentage=[nxc3x97(xcfx80R2/4)/S]xc3x97100 (%).
The camera-ready copy sheet according to the first aspect of the present invention can effectively prevent positional offset of lithographic printing plates even if the large particles that constitute the projections of a predetermined shape on the surface of the sheet are as few as noted above. If they are urged against the back side of the lithographic plate, the latter is sufficiently depressed that the plate with which lithographic printing is being performed on a press can be positively prevented from being positionally offset on a plate cylinder primarily under the printing pressure.
As a further advantage, the camera-ready copy sheet is designed to urge against the back side of the printing plate in only limited areas so that unevenness in printing that may occur can be reduced to an amount that cause no problems in practical operations.
In the first aspect of the present invention, the back side of the lithographic plate may be depressed at the time when the camera-ready copy sheet is positioned between the lithographic plate and the plate cylinder while both the plate and the copy sheet are wrapped around the cylinder. Alternatively, it may not be until the printing pressure is applied after the copy sheet was positioned that the back side of the plate is depressed.
The lithographic printing plates that can be used in the present invention are not limited to any particular types and may be exemplified by common PS plates, plates having a silver diffusion type light-sensitive layer, and those prepared by electrophotographic platemaking processes.
According to a second aspect of the present invention, in a camera-ready copy sheet to be inserted between a plate cylinder and a lithographic printing plate at least the back side of which is made of a non-metallic material, said copy sheet having asperities of a predetermined shape on the front side that are urged against the back side of the lithographic printing plate to depress it, the asperities being formed of projections that consist of at least two groups of particles; a larger particle group having a particle size larger than an intermediate between maximum and minimum sizes have an average size at least twice an average size of particles of a smaller particle group having a particle size smaller than said intermediate, the larger particle group have a higher hardness than the smaller particle group, and the larger particle group forms projections with a height in the range of 5-50 xcexcm.
In the second aspect of the present invention, the sum per unit area of maximum cross-sectional areas of planes in the larger particle group that are parallel to the surface of the sheet preferably preferably ranges from 0.05% to 4% of the unit area. This range is preferably in the range of 0.05 to 3.14%.
In the second aspect of the present invention, if the projections that compose the asperities on the camera-ready copy sheet of the invention are formed of the larger and smaller particles having different hardness values, the larger particles may be fine organic particles such as glass beads, grit particles and particulate inorganic compounds that have an average size of 5-50 xcexcm and a Durometer hardness of at least 100, whereas the smaller particles may be polymer particles such as polyethylene particles that are less hard than the larger particles, typically having a Durometer hardness of 54-72. The smaller particles may be used in suitably selected amounts but from the viewpoints of cost and production efficiency, their use is preferably minimized.
The xe2x80x9cDurometer hardnessxe2x80x9d as used herein is a hardness value obtained by measurement with Type A of a spring-loaded hardness meter called Durometer and manufactured by Shore, Inc., USA.
In the second aspect of the present invention, if the larger particles have an average size of 5-50 xcexcm, the smaller particles have an average size of no more than 25 xcexcm and if the larger particles have an average size of 5 xcexcm, the smaller particles have an average size of no more than 2.5 xcexcm.
In the second aspect of the present invention, the amount of the smaller particles as relative to the larger particles is preferably in the range of xc2xd to 1000 times, more preferably 1-200 times, and most preferably 1-25 times, the amount of the larger particles.
In the second aspect of the present invention, if the larger particles are distributed coarsely to occupy 0.05-4% of unit area as set forth above, a comparatively small number of the particles suffice to achieve positive prevention of positional offset of lithographic plates on the plate cylinder. The use of the smaller particles which are less hard than the larger particles provides ease in separating adjacent camera-ready copy sheets in a stack.
To compute the occupied area percentage as defined in the preferred embodiment of the invention, the surface of a sample is photographed with an optical microscope from right above, the number (n) of projecting particles in a predetermined area S (xcexcm2) is counted, and the occupied area percentage is calculated from the average diameter R (xcexcm) of the particles by the following formula:
Occupied area percentage=[nxc3x97(xcfx80R2/4)/S]xc3x97100 (%).
The camera-ready copy sheet according to the second aspect of the present invention can effectively prevent positional offset of lithographic printing plates even if the large projections that constitute the asperities of a predetermined shape on the surface of the sheet are as few as noted above. If they are urged against the back side of the lithographic plate, the latter is sufficiently depressed that the plate with which lithographic printing is being performed on a press can be positively prevented from being positionally offset on a plate cylinder primarily under the printing pressure.
In the present invention, the back side of the lithographic plate may be depressed at the time when the camera-ready copy sheet is positioned between the lithographic plate and the plate cylinder while both the plate and the copy sheet are wrapped around the cylinder. Alternatively, it may not be until the printing pressure is applied after the copy sheet was positioned that the back side of the plate is depressed.
The lithographic printing plates that can be used in the invention are not limited to any particular types and may be exemplified by common PS plates, plates having a silver diffusion type light-sensitive layer, and those prepared by electrophotographic platemaking processes.
According to a third aspect of the present invention, in a camera-ready copy sheet to be inserted between a plate cylinder and a lithographic printing plate, said copy sheet having asperities of a predetermined shape on the front side, the asperities being formed of projections that comprising at least two particle groups; a larger particle group having a particle size larger than the intermediate between maximum and minimum sizes have an average size at least twice an average size of a smaller particle group having a particle size smaller than said intermediate, the larger particle group have a Durometer hardness of 65 or less which is lower than that of the smaller particle group, the larger particle group forms projections with a height in the range of 5-200 xcexcm; and the sum per unit area of maximum cross-sectional areas of planes in the larger particle group that are parallel to the surface of the sheet ranges from 0.1% to 4% of the unit area.
In the third aspect of the present invention, the sum per unit area of maximum cross-sectional areas of planes in the smaller particle group that are parallel to the surface of the sheet preferably ranges from 0.1% to 99.9% of the unit area.
The size of the smaller particles is preferably in the range of 1.0-100 xcexcm, more preferably in the range of 1.0-25 xcexcm.
The larger particles have a Durometer hardness of no more than 65 and because of their viscoelasticity, they produce great enough friction with the back side of the camera-ready copy to prevent positional offset of the printing plate.
Since they are harder than the larger particles, the smaller particles adhere tightly to the base of the camera-ready copy and, at the same time, they surround and stick to the larger particles to prevent deformation of the latter; in addition, the smaller particles prevent the larger particles from being dislodged during printing.
The camera-ready copy sheet of the invention has projections of different sizes and hardness values provided on the surface and features not only high resistance against uneven printing that is manifested as spots of ink smudge but also low cost and good production efficiency.
In the present invention, the sum per unit area of maximum cross-sectional areas of planes in the first group of particles that are parallel to the surface of the sheet ranges from 0.1% to 4% of the unit area, preferably from 0.1% to 3.14% of the unit area. In the following description, the proportion of unit area that is occupied by the sum per unit area of maximum cross-sectional areas of planes in the particles that are parallel to the surface of the sheet is sometimes referred to as xe2x80x9coccupied area percentagexe2x80x9d.
In the present invention, the projections being comprised of the second group of particles preferably occupy 0.1-99.9% of the unit area.
The xe2x80x9cDurometer hardnessxe2x80x9d as used herein is a hardness value obtained by measurement with Type A of a spring-loaded hardness meter called Durometer and manufactured by Shore, Inc., USA.
The projections that compose the asperities on the camera-ready copy sheet of the invention consist of two or groups of particles, the larger ones being fine inorganic particles such as glass beads or fine particles of comparatively hard polymers such as polystyrene. The smaller particles may be of the same materials as mentioned above; although they may be used in suitably selected amounts, their use is preferably minimized from the viewpoints of cost and production efficiency.
The amount of the smaller particles as relative to the larger particles is preferably in the range of xc2xd to 1000 times, more preferably 1-200 times, and most preferably 1-25 times, the amount of the larger particles.
If the larger particles are distributed coarsely to occupy 0.1-4% of unit area as set forth above, a comparatively small number of the larger particles suffice to achieve positive prevention of positional offset of lithographic plates on the plate cylinder.
To compute the occupied area percentage as defined herein, the surface of a sample is photographed with an optical microscope from right above, the number (n) of projecting particles in a predetermined area S (xcexcm2) is counted, and the occupied area percentage is calculated from the average diameter R (xcexcm) of the particles by the following formula:
Occupied area percentage=[nxc3x97(xcfx80R2/4)/S]xc3x97100 (%).
The camera-ready copy sheet of the invention can effectively prevent positional offset of lithographic printing plates even if the large particles that constitute the projections of a predetermined shape on the surface of the sheet are as few as noted above; if they are urged against the back side of the lithographic plate, their viscoelasticity creates sufficient friction to make them adhere tightly to the lithographic printing plate. As a result, the plate with which lithographic printing is being performed on a press can be positively prevented from being positionally offset on a plate cylinder primarily under the printing pressure.
In the present invention, the camera-ready copy sheet may be urged against the back side of the lithographic plate at the time when it is positioned between the lithographic plate and the plate cylinder while both the plate and the copy sheet are wrapped around the cylinder. Alternatively, it may not be until the printing pressure is applied after the copy sheet was positioned that it is urged against the back side of the plate.
The lithographic printing plates that can be used in the invention are not limited to any particular types and may be exemplified by common PS plates, plates having a silver diffusion type light-sensitive layer, and those prepared by electrophotographic platemaking processes.